People that want seriously random numbers use radioactive decay because the underlying physical phenomena (described via quantum mechanics) is fundamentally random and cannot predict the time, energy, and direction of decay all at the same time (as far as my limited understanding is aware).
>underlying physical phenomena (described via quantum mechanics) is fundamentally random
This is one interpretation of QM. There are many theories explaining eigenbasis collapse (the "random-looking" phenomenon), and some of them are deterministic.
The important thing here is that the data stream is hard or impossible to predict. It doesn't have to be truly random, because we don't even know if "truly random" is a sensible thing to ask for. What we're actually hoping is that you can't predict the result of a measurement, whether or not it's because of randomness or something else.
Most RNGs also don't rely on the uncertainty principle (or, more generally, non-commutative measurements), but instead on e.g. radioactive decay. This is an eigenbasis collapse mechanism (with the two eigenstates being "this thing hasn't decayed" or "this thing has decayed"), but it's not based on uncertainty. This is easier to build physically.
> cannot predict the time, energy, and direction of decay all at the same time
The uncertainty principle isn't the aspect of quantum mechanics that is used in these RNGs. They only need the fact that they can perform an action (measurement of some observable) whose outcome is not predictable. They do not attempt to measure different observables, since this would not help produce randomness. It is in essence just a (theoretically ideal) dice roll.
Here's a great example of radioactive decay in use as a source of entropy[0]. Essentially, each component of a nuclear weapon derives its own private key at time of assembly from the radiation of the weapon itself, and saves the public keys of all the other components; thereafter, it'll only respond to signed communication from those other components.
Basically, it removes the possible attack vector of replacing any command-and-control circuitry with your own, suborned, components.
